Earthworms are well-known inhabitants of healthy soil ecosystems. These segmented creatures perform beneficial functions, including breaking down organic matter, cycling nutrients, and aerating the soil through their burrowing activities. However, their presence becomes a functional and aesthetic problem in finely manicured lawns when their subterranean work results in surface castings. These small mounds of excreted soil damage the turf surface, making mitigation necessary for homeowners seeking a uniform landscape. The methods used to manage this activity focus not on eradication, but on making the surface environment less appealing.
Why Earthworms Become a Lawn Nuisance
Earthworm castings are the waste product of the worm’s digestive process, consisting of fine, mud-like soil and processed organic material. The nuisance arises because certain types of earthworms, specifically anecic species like the common nightcrawler (Lumbricus terrestris), create permanent vertical burrows and deposit their waste on the surface. While most earthworm species void their excrement underground, these few casting species cause visible surface disruption.
The appearance of these muddy mounds is particularly prevalent during periods of high soil moisture, typically in the autumn and early spring, or after heavy rainfall. This surface accumulation creates an uneven texture across the lawn, which interferes significantly with routine turf maintenance. The small piles of soil can dull the blades of a lawnmower and, when flattened by foot traffic or machinery, smear across the grass surface.
This smearing action is particularly detrimental because it can smother the underlying turfgrass, creating bare spots that invite weeds and moss to colonize the lawn. The unsightly lumps interfere with the overall uniformity of the turf. The goal is to discourage the surface casting behavior that compromises the lawn’s aesthetic and health.
Changing Soil Conditions to Discourage Activity
One of the most effective strategies for reducing casting activity involves manipulating the soil’s physical and chemical environment to make it less hospitable to the worms. Earthworms are strongly attracted to organic material, which serves as their primary food source. Therefore, reducing the food supply is a primary cultural control method, achieved by ensuring grass clippings are collected or “boxed off” rather than mulched back into the lawn.
Limiting the use of organic soil amendments and slow-release fertilizers, which provide a rich food source, will also make the area less attractive. Earthworm activity is closely tied to soil moisture, with casting increasing significantly in poorly drained or wet soils. Regulating irrigation to avoid overly saturated conditions and improving sub-surface drainage will encourage the worms to remain deeper in the soil profile.
Soil acidity is another factor that influences earthworm presence, as they prefer neutral to alkaline conditions. Applying acidifying materials can temporarily deter casting activity, though this must be done carefully to avoid harming the turfgrass, which also has specific pH requirements. Products containing ammonium sulfate or iron sulfate can lower the surface pH, providing a less welcoming environment for the casting species.
A physical manipulation involves top dressing the lawn with a layer of medium-coarse, sharp sand. This practice is often employed by turf managers because the sand is thought to irritate the worms’ skin, discouraging them from coming to the surface. The sand also helps to break up and disperse any casts that do form, making them less noticeable and reducing the smearing effect on the turf.
Physical and Chemical Control Measures
Managing existing castings is a purely physical process that requires patience and timing. When castings are dry, they can be easily broken up and dispersed across the lawn using a brush, a rake, or a specialized drag mat. It is necessary to wait for the mounds to dry completely before attempting dispersal, since brushing or mowing wet casts will smear the muddy soil across the turf, suffocating the grass and creating an ideal seedbed for weeds.
Certain non-pesticidal irritants can be used to elicit a temporary response from the worms. For instance, some organic products contain saponins derived from crushed tea seed pellets, which disrupt the earthworm’s mucous layer. These solutions are not lethal but can irritate the worms enough to cause them to surface, allowing for manual removal or simply deterring them from the area for a short period.
When considering chemical control, homeowners must recognize that there are few, if any, legal and effective residential pesticides specifically labeled to control earthworms. Most countries have regulatory restrictions on products targeting earthworms due to their overall ecological benefits and the high risk of environmental damage. While some older insecticides, such as carbaryl, or certain fungicides, like thiophanate-methyl, are known to be toxic, they are not intended or legally approved for casting control in residential settings.
The general consensus among turf professionals is to avoid using chemical agents for earthworm control. These products harm beneficial soil microbes and other non-target organisms, making cultural practices the most reliable and environmentally responsible approach for managing casting in a yard.